In a hybrid vehicle or an electric vehicle, an engine is limitedly used or not used. Thus, engine cooling water required for heating a vehicle room may be insufficient or may not be obtained.
Accordingly, an air conditioning system for a hybrid vehicle or an electric vehicle (hereinafter generally referred to as a “motor vehicle”) employs a heating device capable of heating a vehicle room without engine cooling water. As one example, there is available a heating device that makes use of a positive temperature coefficient (PTC) heater.
As shown in FIG. 1, the heating device using the PTC heater includes a PTC heater 5 installed in an internal path 3 of an air conditioner case 1 and a control unit 7 configured to control the PTC heater 5. The PTC heater 5 is a high-voltage heater, the heat generation amount of which is adjusted by the control of a pulse width modulation (PWM) duty ratio. The PTC heater 5 heats the air blown into a vehicle room, thereby heating the vehicle room.
In a heating mode, the control unit 7 sets a target heating temperature based on an ejection air temperature of a vehicle room inputted from an ejection air temperature detection sensor 7a and applies a PWM duty ratio corresponding to the target heating temperature to the PTC heater 5, thereby controlling the heat generation amount of the FTC heater 5. Thus, the PTC heater 5 is controlled in conformity with the target heating temperature.
If the temperature and volume of the air introduced into the air conditioner case 1 is changed and if the ejection air temperature of the vehicle room is changed, the control unit 7 detects the changed ejection air temperature with the ejection air temperature detection sensor 7a and resets the target heating temperature based on the detected ejection air temperature.
The PWM duty ratio corresponding to the reset target heating temperature is re-applied to the PTC heater 5 to control the heat generation amount of the PTC heater 5, whereby the heat generation amount of the PTC heater 5 is controlled in conformity with the reset target heating temperature.
In the conventional air conditioning system mentioned above, the target heating temperature of the PTC heater 5 is set based on the ejection air temperature of the vehicle room. Thus, the target heating temperature is reset only when the ejection air temperature of the vehicle room undergoes a change.
There may be a case where the volume and temperature of the air passing through the PTC heater 5 is abruptly changed. For example, there may be a case where the volume of the air passing through the PTC heater 5 is changed by the rotation speed level of a blower 9 being changed by a user. In this case, the target heating temperature is not immediately reset even if the heating temperature of the PTC heater 5 is abruptly changed. The target heating temperature is reset only when the ejection air temperature of the vehicle room shows a change.
Thus, the time of resetting the target heating temperature in response to the change of the heating temperature of the PTC heater 5 is delayed. This may delay the control of the heating temperature of the FTC heater 5 and may impair the rapidity of control of the vehicle room temperature.
More specifically, as shown in FIGS. 1 and 2, if the volume of the air passing through the PTC heater 5 is reduced as the rotation speed level of the blower 9 is lowered by a user, the heating temperature of the PTC heater 5 controlled in conformity with a first target heating temperature is abruptly increased. Despite the abrupt increase of the heating temperature of the PTC heater 5, a second target heating temperature is not immediately set. Thus, the resetting of the second target heating temperature is delayed. After a specified time (Δt1) is elapsed, the second target heating temperature is set at the time point when the ejection air temperature of the vehicle room is increased. This poses a problem in that the time of resenting the target heating temperature in response to the change of the heating temperature of the PTC heater 5 is delayed.
This may delay the control of the heating temperature of the PTC heater 5 and may consequently impair the rapidity of control of the vehicle room temperature. As a result, the comfort within the vehicle room is reduced.
Even if the second target heating temperature is set, the heating temperature of the PTC heater 5 continues to increase due to the thermal inertia (see “X” in FIG. 2). Owing to this temperature rise, a long period of time (Δt2) is required for the heating temperature of the PTC heater 5 to reach the second target heating temperature.
As a result, it becomes difficult to control the heating temperature of the PTC heater 5. Consequently, a large amount of air having an undesired temperature is blown into the vehicle room, thereby making the vehicle room unpleasant.